Edited to add: You could ask to buy some in the buy/sell/trade board here too!
Second edit: I totally forgot to mention stores.sheltonpacific.com for some nice koa!
Peteskoacustoms.com has a sale right now but it seems like he's cleared out of a lot of the really nice stuff.
Kiranskoa.com has a few nice blocks right now and he probably has a bunch not pictured on his site to offer you directly if you contact him.
Rockinblocks on Instagram always has beautiful koa, but I don't know how to contact him other than through the Instagram app.
Burlsource.us will usually have really great koa when he has it in stock, but he's out right now. There is some super nice Tasmanian blackwood on the site that is very similar in look to curly koa.
Greenbergwoods.com has some curly koa right now but not the high grade stuff. He also has some really great looking curly Tasmanian blackwood and ringed gidgee that are similar in look to curly koa.
Somebody correct me if I'm wrong, but I think Tasmanian blackwood and ringed gidgee are probably more environmentally friendly than koa as they grow quicker? It's crazy the amount of awesome and more environmentally friendly woods that they have in Australia.